A Canadian tourist's nude swim and nap at at a New Zealand beach ended badly when he woke with a swollen penis after apparently being bitten by a poisonous spider, a report said Friday.

The 22-year-old man woke from his post-swim nap to find his penis swollen and painful and bearing a red mark, a report in the New Zealand Medical Journal said.

By the time he got to hospital in the far north of the country, his penis was severely swollen, his blood pressure was high and his heart racing.

Chest pains and other symptoms developed and it was assumed he had been bitten by a katipo, an uncommon beach-loving spider - whose bite is sometimes fatal - which is related to Australia's redback and North America's black widow.

The man was treated with anti-venom and his condition improved although he suffered from heart inflammation and required a total of 16 days in hospital before being released.

"It was a rather nasty, ill-placed bite," said doctor Nigel Harrison of Whangarei Hospital.